1. Field of the Invention
The disclosed embodiments of the present invention relate to an oscillation circuit, and more particularly, to a method for detecting frequency offset of an oscillator, and an associated circuit.
2. Description of the Prior Art
FIG. 1 is a chart illustrating an oscillation frequency output by a crystal oscillator with respect to temperature. When there is a change in temperature, the oscillation frequency will inevitably deviate from a target frequency. As shown in FIG. 1, the curve of the oscillation frequency versus the corresponding temperature is S-shaped, i.e. an S curve. The offset of the oscillation frequency affects electronic devices, especially for systems or applications which have a low tolerance for frequency offset, such as Global Positioning Systems (GPS).
A conventional compensation method for frequency offset is the application of a temperature compensated crystal oscillator (TCXO). When the TCXO is manufactured, an S curve of a crystal oscillator is derived and saved in an external chip in advance. The chip can further compute an inverse curve against the S curve and generate a model accordingly, to exactly compensate the frequency offset of the crystal oscillator. The TCXO is uneconomical in manufacturing terms, however, and if the S curve of the crystal oscillator changes, the external chip may fail to compensate the frequency offset precisely with repeated use, as the stored S curve is a fixed curve. In other words, the conventional TCXO has a limited lifetime. Another prior art compensation method measures the S curve and then compensates by utilizing external digital circuits. This method has the shortcomings of a complicated control mechanism, and also has a limited lifetime.
Therefore, there is a need for a novel frequency offset detection mechanism capable of computing frequency offset of a crystal oscillator without deriving an S curve of the crystal oscillator in advance, and compensating the crystal oscillator based on the computed frequency offset.